Process for producing within electron tubes,in particular television picture tubes,a thin metallic film capable of sorbing their residual gases



June 18. 1968 P. DELLA PORTA ETAL 3,388,955

PROCESS FOR PRODUCING WITHIN ELECTRON TUBES, IN PARTICULAR TELEVISIONPICTURE TUBES, A THIN METALLIC FILM CAPABLE OF SORBING THEIR RESIDUALGASES Original Filed Aug. 24, 1965 2 Sheets-Sheet 1 INVENTORS PAOLODELLA PORTA TIZIANO A. moRm EL-lO RABUSIN M, EM

TTORNEYS June 1 IN PARTICULAR TELEVISION PICTURE TUBES A THIN METALLICFILM Original Filed Aug. 24, 1965 CAPABLE OF SORBING THEIR RESIDUALGASES 2 Sheets-Sheet 2 V(cm sec") 2 F Qt (cm-K to") 80% fi 7 1 50% I E Ii j l 40% [A I I 20% I i I I 5 5', 5' 12 16 T} 20 +2 24 (st-:c.)

INVENTORS PAOLO DELLA DRTA TILIANO A. GIIORGI ELIO RABUSIN United StatesPatent 3,388,955 PROCESS FOR PRODUCING WITHIN ELECTRON TUBES, INPARTICULAR TELEVISION PICTURE TUBES, A THIN METALLIC FILM CAPABLE OFSORBING THEIR RESIDUAL GASES Paolo Della Porta, Milan, Tiziano A.Giorgi, Rho, Milan, and Elio Rabnsin, Milan, Italy, assignors to S.A.E.SGetters S.p.A., Milan, Italy, a company of Italy Original applicationAug. 24, 1965, Ser. No. 482,104. Divided and this application Aug. 10,1967, Ser. No. 664,593 7 Claims priority, application Italy, Feb. 25,1965, I

1,708/65, Patent 742,042 9 Claims. (Cl. 316--25) ABSTRACT OF THEDISCLOSURE A process of producing within a picture tube, having wallsand a screen, a thin getter metal film for sorbing the residual gases,comprising evaporating the getter metal in the picture tube in thepresence of a gas present in an amount such that the mean free path ofthe getter metal atoms in the gas is equal to or less than the distancebetween the metal evaporating source and the tube screen.

This application is a division of application Ser. No. 482,104, filedAug. 24, 196 5.

This invention relates to a process for producing within electron tubes,and in particular television picture tubes, a thin metallic film capableof sorbing their residual gases and characterized by the fact that itsdistribution is substantially limited to the desired surfaces of thetube and also by the fact that the film on the screen of the televisionpicture tube is of reduced thickness, when compared to otherconventionally produced films, and so presents less impedance to theflow of electrons directed at the phosphors on the screen of the tube.

The present invention also relates to the device to be employed toaccomplish the above mentioned process.

It is well known that, in order to maintain the required degree ofvacuum in, for example, television picture tubes, as well as in manyother types of electron tubes, the generally used technique is to employan internal chemical pump or getter. The getter consists of a thinmetallic film deposited on appropriate surfaces of the device after ithas been processed and isolated from the conventional pumps. This thinmetallic film is usually barium and is commonly deposited from a gettercontained which may be heated by externally induced radio frequencycurrents. The getter container consists of a non-magnetic stainlesssteel ring of U-shaped section in which is usually compressed in equalproportions a mixture of powdered 50% barium-50% aluminum alloy andnickel. On heating, as previously mentioned by externally induced radiofrequency currents, an exothermic reaction occurs between thebarium-aluminium alloy and the nickel when the temperature is about 800C. causing an instantaneous and spontaneous increase in temperature upto above 1300 C. with the consequent evaporation of 30 to 40% of thebarium within the container. Since, however, the radio frequency poweris still applied to the getter container the remaining barium alsoevaporates but at a much decreased rate. Another type of getter is alsoknown where no such reaction occurs due to the absence of the nickel andas a consequence this type of getter is called endothermic. Its maindisadvantage is the irreproducibility of yield and as a consequence itis rarely empolyed in manufacturing processes. The barium film obtainedfrom either of these two processes is very active chemically, it reactswith the residual gases present in the tube and efiiciently removes themfrom the gaseous phase. Furthermore, the action of the barium film isnot instantaneous or short lived, but by means of diffusion phenomena itis capable of continuing itsbgettering or pumping action throughout thelife of the tu e.

It is well known that the rate of reaction of the residual gases in thetube with the barium film increases proportionally with the lattersincrease in surface area. It is for this reason that in electron tubes,and in particular in television picture tubes, the greatest possibleinternal surface area is utilized for film deposition. This naturallyresults in the complete coverage of the screen of the television tubewith barium. In fact due to the position of the getter container, whichis mounted on the top of the electron gun which is in turn situated onthe normal to the centre of the screen, and as a result of the wellknown distribution laws of evaporation, a substantial fraction of thebarium evaporated is deposited on the screen of the television picturetube. Its distribution is circularly symmetrical and is a maximum at thecentre of the screen and a minimum on its periphery.

The image on the screen of a television picture tube is due to the highenergy electrons which impinge and excite suitable phosphors, evenlydistributed on the internal surface of the screen. To increase theluminescence of these phosphors in the forward direction it is standardpractice to cover them, on the side from which the electrons areincident on them, by a thin layer of aluminium which has also otherelectrical functions connected with the potential distribution withinthe tube, the protection of the phosphors from ion burn, etc. Thepresence of this aluminium film causes a loss in energy of the electronswhich are directed at the phosphors and so decrease their luminosity.The same effect is also caused by the barium present on the screen andalthough its thickness is much less than that of the aluminium itseffects are more pronounced due to its higher molecular mass. Aspreviously mentioned the thickness of the barium film is greatest at thecentre of the screen and it is here that electrons are decelera-ted to agreater extent. This sometimes causes the characteristic darker centralportion in the image on the screen and can only be avoided by usinghigher electron accelerating potentials in the design of the set withconsequent increase in production costs.

The advantages which would result in television tube production andusage, and in similar electron devices, if the barium film on the screencould be rendered more uniform and thinner are obvious. In fact if sucha solution were possible a very appreciable economic saving would resultin the production of television sets and similar devices. If such asaving were not required advantage could be taken from the reducedbarium film thickness to improve the quality of the tube. Some casesexist where tubes are produced without aluminium backing of thephosphors and in these cases no barium must be deposited on thephosphors since they would be damaged.

Hence in all the above mentioned cases the use of a suitable device andtechnique for efficiently controlling the barium deposition on thescreen of the tube would be extremely useful for the industry as awhole.

The methods employed heretofore for controlling the barium filmthickness on the screen of television picture tubes, and similarelectron devices, have relied principally in mechanically directing theissuing barium vapour by means of various types of deflecting bafiiesfashioned on the getter container. In one particular embodiment of thisprocedure the baffies are such as to direct the barium vapours towardsthe centre of the bulb, i.e., on the normal to the centre of the screenof the tube. The coverage being so adjusted as to cause collisionsbetween the barium atoms before they reach the screen and thus cause avirtual point source of barium atoms from which they may evaporate inall directions. In still another embodiment using mechanical directionof the barium vapours the container of the getter has battles whichdirect the evaporating barium atoms in a direction perpendicular to thatof the normal to the centre of the screen. Various other solutionsbetween these two extremes have also been suggested and adopted.However, their major drawback has been their low efficiency or ifefficient the fact that they limit to a very great extent the surfacearea of the film which they produce and thus render it ineffective forthe purposes required. The only virtue of some of these methods ofmechanically directing the barium evaporation, however inefiiciently,has been to reduce the quantity of barium refiected towards the electrongun. In fact barium evaporated in this direction can be extremelytroublesome since it can give rise to secondary emission effects, shortcircuits, etc.

Thus, the principal object of the present invention is to provide ameans of reducing the quantity of barium reaching the screen of atelevision picture tube or similar device, if required even to zero, andto redistribute the excess barium on other internal surfaces which haveup till now been less utilized, for example the cone area. According tothe present invention attempts are not made, as occurs with knownconventional methods, to direct the barium atoms by means of bafiles,but steps are taken to prevent large amounts, or even all of the barium,from reaching the screen of television picture tubes or similar devicesbyintroducing a deviating or retarding mass in its way. As will becomeevident subsequently the invention relates essentially to exothermictype getters.

The process, according to the invention, consists in providing a getterdevice which in spite of its inherent high rate of evaporation, and justbecause of this characteristic, provides within the volume in which itis evaporated a suitable deviating and retarding mass which avoids theotherwise pre-dominant deposition of getter material in the forwarddirection.

Thus it is a characteristic of the invention that the evaporation of thegetter occurs in the presence of a suitable gas having an adequatemolecular mass and present at a predetermined pressure so that the meanfree path of the barium atoms in it is smaller than the distance betweengetter container and screen. It is also a characteristic of theinvention that the gas used is repumped by the actual barium film,without however, consuming but a very minute fraction of the filmsgettering capacity, in a very short interval of time. In fact it may bestated that the last barium to leave the getter container does not findin its way any gas and not being hindered can be deposited according tothe known distribution laws. However, if no barium is required on thescreen the heating of the getter container can be stopped immediatelyafter having initiated the exothermic reaction. Further characteristicsof the present invention are that the gas employed be nitrogen,introduced into the tube in the form of a stable compound, which willdissociate only at temperatures immediately below the onset of theexothermic reaction and that the gas be present in the tube before theonset of such exothermic reaction.

Processes are already known whereby continuous evaporation of materialsin an inert gas atmosphere is utilized for the production of powderscharacterized by very fine particle size. However, it must be realizedthat in television picture tubes and similar electron devices such fineparticles must be avoided at all costs and that, therefore conditionsleading to such phenomena must be avoided.

The novelty of the invention rests in the fact that, a suitable choiceof gas and pressure enables the control of up to 50% of the barium whichmay be obtained from an exothermic type getter without producing anyfine particles and without leaving a high pressure in the tube at theend of the evaporation process. Thus this fraction of the barium isdeposited only in the cone and neck zones of the television picturetube, whilst the rest of the barium is evenly distributed on these andthe remaining internal surface areas of the tube. Should no barium berequired on the screen this result can also be obtained by interruptingthe heating of the getter container once being initiated the exothermicreaction In addition to rendering possible a fine control of thedistribution if the barium film, the use of getters according to theconcepts of the present invention produces barium films, as alreadyknown in the art, characterized by a high porosity and so increasesappreciably the real surface of the film produced. Practical tests haveshown that this increase in specific surface area involves an increaseof sorption characteristics by a factor of 2 to 3. This fact presentsobvious advantages from the point of view of tube life or from aneconomic stand point. Such porous films are a result of submicroscopicglobule formation in the vapour phase which condense as such on thewalls of the vessel.

To put the inventive process into practice any gas may at first sightappear suitable. However, the following points should be borne in mindwhen trying to decide on the nature of the gas to be employed. Itsretarding and defleeting action increases with:

(a) increase in molecular mass,

(b) increase in pressure,

(c) decrease in chemical affinity to barium.

Another point to observe as far as regards (b) is that the pressure usedshould not be too high since it could cause a serious decrease ingettering capacity of the getter film.

On the basis of the above considerations the rare gases such as argon,krypton, etc. would seem very attractive. However, these gases are notsorbed by the getter material and so if the getter film is deposited, asoccurs usually, in the closed tube the rare gases would remain in itrendering its functioning impossible. Therefore such gases may not beutilized for the purposes of the present invention. Other gases such ashydrogen, carbon monoxide, carbon dioxide and oxygen may be considered.However, such gases would result extremely detrimental to tubecharacteristics as such and also because they tend to producehydrocarbons, water vapour, etc. which are harmful to cathode activity.Not only, but all these gases would be contrary to one or more of thethree points previously indicated. In fact for hydrogen a high pressurewould be necessary due to its particularly low mass. For the other threegases a high pressure would be necessary to cope with the extremely highreaction rates which they present with pure barium films.

The gas which to the ends of the present invention has resulted as themost suitable is nitrogen. In fact this gas does not damage the cathode,does not produce undesirable side products, it has a relatively highmass, it is not exceedingly reactive with barium, although it is easilysorbed by it, and as a consequence the pressure necessary to obtain thedesired effect is rather low. The nitrogen pressure which has proven tobe most satisfactory for the scope of the present invention is of theorder of between 5 X 10 and 1X10 torr. Pressures above 5x10- torr usingup too great a fraction of the barium film and pressures below 10 torrnot being sufiicient to the scope of the present invention.

The introduction of the selected gas into the tube may occur in a numberof ways. One of these could be to introduce the required pressure in thetube whilst still on the pump and just before tip-01f. However, such amethod would not be technically economical. Hence, we propose tointroduce the gas into the tube by utilizing a compound of nitrogenwhich will be dissociated by heating prior to evaporation of the barium.Such a compound could be in powdered form and be mixed in the correctproportions with the getter alloy in the container, it could also bemounted on a separate support removed from the getter, and it may evenbe that the actual getter container has been subjected to anitrogenating action giving rise to suitable compounds. However, thecompound employed must be stable to all possible aging and pretreatmentssuch as deionized water wash, drying and vacuum heat treatments up to400 C. The compound used should, if mounted in the getter container,dissociate at temperatures below the one at which the exothermicreaction sets in. The use of barium azide, for example has been longknown, with other finalities of those explained in the presentinvention, however, barium azide would not be usable since its extremeinstability to aging under normal atmosphere conditions or to heatingunder vacuum are well known.

Nonlimiting examples of compounds suitable to this end in the presentinvention are those obtained from nitrogen and any of the followingmetals or alloys thereof: nickel, iron, molybdenum, manganese, titanium,zirconium, hafnium, vanadium, niobium, tantalum, chromium, tungsten,cobalt, silicon and stainless steel.

'As previously mentioned the nitrogen bearing compound may be mixed withthe exothermic charge in the getter container, it may be physicallyseparated from it and thus the gas it contains may be evolved a prioribefore heating the getter container or the getter container itself maybe nitrogenated. Theprinciple of the invention, although essentially forexothermic getters, cannot be excluded from use in endothermic getters.However, since in this case the evaporation is initially low and. thenpicks up speed the quantity of gas must be initially low and must slowlyincrease as the rate of'evaporation rises. In the case of exothermicgetters since, as previously mentioned, 30 to 40% of the barium isemitted simultaneously and at the same time, the nitrogen pressurepresent has full effect on this quantity of barium. The remaining bariumwhich evaporates at a slower rate is less influenced due to the sorptionof nitrogen which has taken place. Nevertheless an appreciable influenceis also exercized on this second quantity of barium.

The exothermic getter, containing the nitrogenous compound mostsuitable, and having any desired or suitable deflecting bafflesarrangement can be mounted as the normal and more conventional getter onthe electron gun of the tube or in any required or desired position. Theusefulness of the bafiles is, however, very limited in the gettersobject of the present invention as far as regards the barium filmdistribution on the screen and on the cone of the television picturetube. Their cation is still useful as far as regards back evaporation ofbarium towards the gun of the tube.

A practical illustrative and nonlimlting example of the invention nowfollows.

An 11 inch television picture tube having a flare angle of 110 is usedin conjunction with a conventional getter mounted 1% from the yokereference line (Y.R.L.) consisting of a powdered mixture of equalproportion of 50% Ba-50% alloy and nickel supported in a stainless steelcontainer of ring shape. The getter is evaporated under good vacuumconditions producing the distribution of barium in the tube indicated inFIG. 1 which shows schematically a section of the tube together with twodiagrams (a) and (b). These diagrams refer to the barium thickness onthe screen (a) and cone (b) sections of the tube. In each diagram thedashed lines, indicated by 1 and 3, refer to the above mentioned type ofgetter. To be noted that the thickness of the film at the centre of thescreen is 1400 Angstrom units. In a similar tube was then mounted agetter of exactly similar characteristic as above but containing also apredetermined quantity of powdered iron nitride (Fe N) such as toproduce in the tube a pressure of about 1 10- torr of nitrogen duringthe normal flashing of the getter and prior to the onset of the bariumevaporation due to the exothermic reaction. In this case the barium filmdistribution in the tube is shown by the continuous lines in thediagrams (a) and (b) of FIG. 1 indicated by 2 and 4. It will be notedthat the 6 thickness of the barium film at the centre of the screen isless than 400 Angstrom units in this case.

The relative quantities of barium on the screen, cone and neck of thetube with conventional getters are respectively 7, 0, 1, 8, 16, 2 mg.and with the getters object of the present invention they arerespectively 2, 5, 3, 5 and 19 mg. Such significant reduction of bariumon the screen is very useful for electron transparence and results in anincreased light output from the tube of up to 30%.

The fact that evaporation of the barium film in a nitrogen atmosphere inno way reduce the characteristics of the barium film produced isillustrated in FIG. 2. In this figure on the ordinates are reported thegettering rates of carbon monoxide whilst on the abscissae are reportedthe related quantities of carbon monoxide sorbed. The television picturetube and getter types as well as position are the same as thosepreviously mentioned. The gettering characteristics where, also in thiscase, measured for a 25 mg. Ba film and the constant CO pressure on thegetter was 5.10- torr. Curve 5 refers to the characteristics of aconventional greater film whilst curve 6 refers to those of a filmobtained according to the present invention. It will be observed thatthe gettering characteristics of the film obtained by evaporating thebarium film in a nitrogen pressure have increased since the amount ofbarium rendered accessible to the gas has been increased appreciably dueto the increase in specific surface area of a film formed in thismanner.

The diagram of FIG. 3 clearly shows the retarding action of the gasintroduced on the forward evaporation of barium. Such curves have beenobtained by using a quartz crystal thickness monitor. The crystal ismounted on a tube similar to the one above at the centre of the screen.Its shift of frequency as barium is deposited on one of its faces is adirect measure of the quantity of barium condensed on the face itself.On the ordinates of this diagram are reported the percentages of bariumthickness referred to the conventional getter at the end of evaporation,on the abscissae is reported the time of evaporation. The external radiofrequency power is applied at zero time (not shown). The starter times Sand S indicated show the instant when the exothermic reaction begins,whilst the total times T and T are the times at which the radiofrequency current to the getter container is discontinued. The curve 7refers to the conventional getter, whilst curve 8 refers to the getterobject of the present invention. It will be clearly observed how the gaspresent in the getter, and liberated in the tube before the startertime, has its principal action essentially during the first few secondsof evaporation although even in the subsequent stages a certain actionis also being exercized. It should however be noted that all the gasintroduced after a few minutes has been repumped by the getter as hasbeen revealed by pressure measurements carried out during these tests.

We claim:

1. A process of producing within a picture tube, having walls and ascreen, a thin getter metal film for sorbing the residual gasescomprising evaporating said getter metal in said picture tube in thepresence of a gas wherein said gas is present in an amount such that themeans free path of the getter metal atoms in the gas is equal to or lessthan the distance between the metal evaporating source and the tubescreen.

2. A process according to claim 1 characterized in that the gas isintroducing as such in the picture tube before the evaporation of thegetter metal.

3. A process according to claim 1 characterized in that, along with themetal to be evaporated, admixed therewith or mechanically separatedtherefrom, a compound capable of evolving the desired gas upon heatingis introduced into the picture tube.

4. A process according to claim 3, characterized in that a compound isused which is not appreciably dissociated during the preliminarytreatment of the tube or during air exposure.

5. A process according to claim 1 characterized in that the gas isnitrogen produced by the thermal decomposition of Fe N.

6. A process of producing within a picture tube, a thin getter metalfilm for sorbing the residual gases comprising evaporating said gettermetal in said picture tube in the presence of a gas having a pressure ofbetween 5 10 torr and 1x torr, wheerby a major portion of the gettermetal is deposited on the internal surfaces of the tube in the vicinityof the getter metal source.

7. In a process for depositing a getter metal on the inside surfaces ofa picture tube, said picture tube having a viewing screen connected toconical walls, said getter metal being deposited in the form of a thingetter metal film by evaporation from a getter metal source, theimprovement comprising evaporating said getter metal in the tube in thepresence of a gas causing collision of the getter metal molecules withthe gas molecules with a consequent deposition of a reduced amount ofgetter metal on the screen and an increased amount of getter metal onthe walls of the tube.

8. A process of producing a thin getter metal film within a picture tubehaving a screen, walls connected to the screen and a neck connected tothe wall, comprising the steps of:

(a) placing in said tube in the vicinity of the neck a getter devicecomprising a getter metal admixed with -a gas releasing material whichreleases its gas at a temperature below the evaporation temperature ofthe getter metal,

(b) evacuating said tube,

(c) releasing the gas from the gas releasing material,

(d) evaporating the getter metal in the presence of the gas causingcollision of the getter metal molecules with the gas moleculesdepositing an increased amount of the getter metal on the walls of thetube and a decreased amount of getter metal on the screen of the tube.

9. A process of producing a thin getter metal film within a picture tubehaving a screen, walls connected to the screen and a neck connected tothe walls, comprising steps of:

(a) placing in said tube in the vicinity of the neck a getter devicecomprising a getter metal admixed with a nitrogen releasing materialwhich, releases nitrogen at a temperature below the evaporationtemperature of the getter metal,

(b) evacuating said tube,

(0) releasing the nitrogen from the nitrogen releasing material,

(d) evaporating the getter metal in the presence of the nitrogen causingcollision of the getter metal molecules with the nitrogen moleculesdepositing an increased amount of the getter metal on the walls of thetube and a decreased amount of getter metal on the screen of the tube.

References Cited UNITED STATES PATENTS 3,121,182 2/1964 Hui 316-3,131,983 5/1964 Harries 316-25 1,861,643 6/1932 Pirani 31625 RICHARD H.EANES, JR., Primary Examiner.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No 3,388,955 June 18 1968 Paolo Della Porta et al.

It is certified that error appears in the above identified patent andthat said Letters Patent are hereby corrected as shown below:

Column. 1, line 67, for "empolyed" read employed column 4 line 7 for"if" read of column 6" line 21, for "greater" read getter line 60, for"means" read mean column 7, line 8, for "l0 read 10' Signed and sealedthis 1st day of July 1969.

(SEAL) Attest:

Edward M. Fletcher, Jr. E. JR.

Commissioner of Pate fits Attesting Officer

